A mobile satellite communication system typically includes at least one satellite, at least one fixed ground terminal such as a gateway, and many mobile terminals. The mobile terminals may communicate with the gateway or other mobile terminals via the satellite over communication channels. In such systems, many carrier signals provide the communication channels for a given geographic area. Each carrier signal occupies a different frequency band within an allocated frequency spectrum. Each band within the frequency spectrum is separated by an inter-channel guard band to avoid interference between channels. Since there is a limited amount of frequency spectrum allocated to the satellite communication system, it is desirable to minimize the inter-channel guard band. The inter-channel guard band can be minimized when each carrier signal is closely maintained at the proper frequency for that channel.
However, there are many sources for frequency errors in the mobile satellite communication system. Such errors include independent frequency reference oscillator errors in each mobile terminal and in each gateway, frequency translation errors at the satellite, and doppler induced frequency errors due to satellite or mobile terminal motion.
A conventional method of providing an accurate frequency reference requires placing a high-stability frequency oscillator in each satellite in the system. In such a method, the mobile terminals and the gateway synchronize to signals received from the satellite. However, placing high-stability frequency oscillators in a satellite is expensive and potentially unreliable. For example, if the satellite high-stability oscillator fails, the cost of fixing the satellite oscillator may be enormous, e.g. the cost of retrieving and fixing the satellite or launching a new satellite. Accordingly, there is a need for a less expensive and more reliable method of stabilizing transmission frequencies in a mobile satellite communication system.
The present invention relates generally to satellite communication systems, and more particularly to a method and system for stabilizing transmission frequencies in a mobile satellite communication system. The present invention allows a single high-stability oscillator to be placed on the ground, and the frequency offsets from other terminals and satellites to be corrected accordingly. The present invention eliminates the need for a placing a high-stability oscillator in remote satellites or terminals.
The preferred method of stabilizing the transmission frequency of a mobile satellite communication system involves placing a high-stability oscillator in a ground-based gateway terminal. Preferably, the high-stability oscillator produces a reference signal to derive all transmit and receive frequencies for the system. Based on the satellite ephemris, the gateway terminal determines the frequency shift introduced on the uplink to the satellite. The gateway then adjusts its transmit frequency to compensate for the shift so that the signal recieved by the satellite is at the reference frequency.
A preferred embodiment of the invention also involves receiving frequency offset information transmitted by the gateway, and adjusting a reference oscillator in a mobile terminal based on the frequency offset information. Preferably, the mobile terminal also transmits a first signal to the gateway, and the gateway measures a mobile terminal frequency offset from the first signal. The gateway preferably transmits the mobile terminal frequency offset to the mobile terminal, and the mobile terminal adjusts its reference oscillator to compensate for the mobile terminal frequency offset.
A preferred embodiment also involves placing a call between the mobile terminal and the gateway, measuring a mobile terminal frequency drift offset at the gateway, reporting the mobile terminal frequency drift offset to the mobile terminal, and adjusting the reference oscillator in the mobile terminal to compensate for the mobile terminal frequency drift offset.
Another preferred embodiment provides a method of stabilizing a communication frequency in a mobile to mobile call placed over a satellite mobile communication system. The preferred method provides a first mobile terminal having a reference oscillator, and a second mobile terminal. A call is placed between the first mobile terminal and the second mobile terminal. A forward control channel is monitored at the second mobile terminal, and messages are transmitted from the first mobile terminal to the second mobile terminal over a traffic channel during the call. A frequency offset in the first mobile is determined by comparing the frequency of the traffic channel received by the second mobile to the frequency of the forward control channel received at the second mobile. The frequency offset to the first mobile is reported, and the reference oscillator is adjusted to compensate for the frequency offset.
The preferred embodiment of the system includes a plurality of mobile terminals, a satellite adapted to communicate with the mobile terminals, and a gateway in communication with the satellite. Each mobile terminal has a mobile terminal reference oscillator, and the satellite has a satellite reference oscillator. The gateway has a high-stability reference oscillator and a digital processor. The digital processor includes a routine for calculating a frequency offset, such as reference oscillator drift. The gateway transmits the frequency offset to at least one of the mobile terminals via the satellite. Preferably, the gateway periodically transmits the frequency offset to each of the mobile terminals. At least one of the mobile terminals receives the frequency offset and adjusts the mobile terminal reference oscillator to compensate for the received frequency offset.
In a preferred embodiment, each satellite communicates with the mobile terminals by transmitting and receiving signals over a plurality of spot beams, each spot beam having a coverage region and at least one control channel. Also the routine in the gateway preferably calculates a plurality of frequency offsets, where each of the calculated frequency offsets is associated with one of the spot beams. Preferably, each frequency offset is calculated based on the position of the coverage region for the associated spot beam. In addition, the gateway preferably transmits the frequency offsets over the control channels to the mobile terminals.
The invention, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.